A System to Prevent Power Outages in Optical Transmission Lines (21073)
This breakthrough method provides the only known practical means by which to predict power outage probabilities and bit-error rates in fiber optic lines caused by "polarization mode dispersion (PMD)." Today’s stringent standards permit only very infrequent power outages, so system designers need to have a way to predict and prevent such outages. The Northwestern invention provides a way to do this. It has been used in the form of software, but potentially can be implemented in hardware.
Competing approaches fall short of estimating outage probabilities and bit-error rates, because it is not feasible to run the large number of simulations or experiments that they require. In contrast, the Northwestern invention has produced simulation speedups of several orders of magnitude over competing techniques, thereby enabling one to obtain results that would otherwise be beyond the capability of competing methods.
Optical fibers do not have perfect properties. Slight irregularities and asymmetries introduced during the manufacturing or installation processes lead to optical fibers with birefringence, a property whereby one particular optical polarization propagates slightly faster than the other. Furthermore, these perturbations vary randomly with distance and over time. The varying fiber properties produce random polarization state changes in a propagating optical signal, which, when combined with the birefringence, lead to signal distortion known as polarization mode dispersion (PMD). PMD can cause outages in optical fiber networks. As the signal-channel bandwidth increases, these outages tend to occur more frequently. As a result, PMD has emerged as one of the major impairments to upgrading current per channel data rates to 10 Gbit/s and beyond in terrestrial wavelength-division-multiplexed (WDM) systems, and is expected to be a significant problem at 40 Gbit/s, even in newly installed fiber.
Today’s standards typically call for no more than 1 minute per year of fading in optical lines due to PMD. As a result, even rare outages can be too frequent by today’s stringent standards. At 40 Gbit/s, these requirements are even harder to meet; PMD-inducing imperfections that currently are tolerable can become unacceptable. This invention calculates the "tail" of the probability distribution curve – those rare instances where actual outages occur. This is obviously the most important part of the curve to calculate, but identifying this portion of the curve is not feasible with known, alternative methods.
The invention presents a method that directly determines PMD-induced power penalties. The method works by:
Targeting specific amounts of first- and second-order PMD, including the rare events where the amounts of first- and second-order PMD are large. Using importance sampling to determine the actual probabilities associated with these targeted PMD events. Directly determining the outages that result at these targeted PMD values. Combining different targeted values to produce the complete range of PMD-induced transmission impairments and an overall outage probability. STATE OF DEVELOPMENT AND IMPLEMENTATION: The method has been implemented in software and has been piloted for one company using that company’s data. A currently available simulation method ran on multiple computers for some months to yield precision of 10-6 probability, whereas the invention ran on a single computer in less than two days to yield precision of 10-25. The invention has been expanded to include multiple algorithms that enable a solution that is robust and easy to implement.
.S. Patent No. 7,068,896
U.S. Patent No. 7,289,708
Gino Biondini, Sarah Fogal, William Kath
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